The present disclosure relates to a hydraulic control device for an automatic transmission that includes a continuously variable speed change mechanism to be mounted on a vehicle, for example.
Automatic transmissions that use a belt-type continuously variable speed change mechanism (hereinafter referred to as a “continuously variable speed change mechanism”) that includes a pair of pulleys and a belt (or a chain) wound around the pulleys and made of metal and that continuously varies a speed by changing the effective diameters of the pulleys have been widespread as automatic transmissions that are suitable for use in vehicles, for example. The continuously variable speed change mechanism includes, as the pair of pulleys, a primary pulley configured to adjust a speed ratio and a secondary pulley configured to adjust a belt holding force.
Such automatic transmissions have linear solenoid valves configured to engage and disengage a plurality of engagement elements such as clutches and brakes provided in a power transfer path to switch a travel mode. As measures for a case where the linear solenoid valves are subjected to an off failure for some reason (hereinafter referred to as “at the time of an off failure”), there is occasionally provided a fail-safe valve configured to supply a source pressure such as a modulator pressure to engagement elements that are required as a minimum for the vehicle to travel forward, for example, at the time of an off failure.
There is known a hydraulic control device that utilizes a primary linear solenoid valve that supplies a primary control pressure for regulating a primary pressure to be supplied to a hydraulic servo for the primary pulley, for example, in order to switch the fail-safe valve at the time of an off failure (see Japanese Patent Application Publication No. 2011-196390). In the hydraulic control device, during normal times, the primary linear solenoid valve regulates the primary pressure utilizing a low-pressure region of the primary control pressure. At the time of an off failure, meanwhile, the primary linear solenoid valve regulates the primary pressure, concurrently with switching the fail-safe valve by supplying an engagement pressure, utilizing a high-pressure region of the primary control pressure.